Process of preparing stabilized synthetic fluoaluminate



Patented Aug-'14;

, UNITED s'l'A'l's-s assess-l rnocass or PREPARING smnnlrzsn sYN'rns'rrc rwoswsmmra Charles B.,Miles, Upper Darby, 1a., asaignor a The Pennsylvania Salt Manufacturing Company, Philadelphia, Pa, a corporation oi Pennsylvania No Drawing. Application l ebruary 9, o Serial No. 475,277

13 Claims.

The present invention relates to a process tor 'the treatment of synthetic fluoaluminates, for example the alkali metal fluoaluminates of the cryollte and chiolite type; and more particularly it relatesto the treatment of synthetic fluoaluminates, characterized by loss-of fluorine upon heating, to convert such fluoaluminates into products which do not lose an appreciable amount of fluorine when subjected to high temperature, for example, in the case of synthetic cryollte, temperatures as high as about 700 C. The term fluoaluminate is defined as the double fluoride of aluminum and another metal, and is exempliiled by cryollte with a formula of NaaAlFs or chlolite with a formula of NasAlsFn. The term double fluoride includes compositions of dissimilar structure; e. g. mixtures or pure compounds wherein there are present as the principal constituents fluorine, aluminum, and one or. more additional metals.

The present application is a continuation-inpart of application Serial Number 430,997, flied employed in the manufacture of aluminum An examination of a number oi synthetic fluoaluminates made in accordance with the usual methods or manufacture has shown that the liberation of fluorine upon heating is a common fault. For example, analyses of sixteen diflerent samples of synthetic cryollte average 86.9% NasAlFs (cryolite) with 4.84% ignition loss at 700 0., the samples yielding detectable acid vapor in substantially every instance. The loss of hydrofluoric or equivalent acid may amount to as much as onehalf of the ignition loss, and this violatile acid may be due to decomposition of hydrofluoaluminic acids or basic or other fluorides or may be generated by autohydrolysis of hydrates of alu-, minum fluoride which may be unavoidably coprecipitated with the fluoaluminate. The bulk of the acid can be liberated only at relatively high temperature, although some-acid may be found at low temperature by titration with alkali.

Consideration of the structure of synthetic fluoaluminates gives a clue to a possible source-oi this relatively volatile acidic impurity.. Indications are that, upon application 01' heat to syn- (Giza-s8) J thetic fluoaluminates oi the type under consideration, a rearrangement or realignment of the ionic constituents occurs resulting in the formation of molecules of free acid.

chains of AlFe octahedra Joined by metallic cations which more or less completely nil, the spaces between the chains. If the fluoaluminate ,is formed in aqueous medium, hydrates are posaluminate. The structures of the hydrates'may be supposed to be quite similar to those oi'the anhydrous materials, but with 11* ions substituting in part for the metallic cations and with OH. groups replacing combined 1' atoms. on this basis,-it is understandable that loss-oi!!!" should of heat. The mobile 5+ ion may unite either with OH or F- ions. The data and theoriesherein discussed are recorded in the interest of claritying the nature of the invention and are not to tion.

.In the interest oi. iurther clariflcation, a dis-. tinction is made within the scope of the present inventionbetween two types of fluorine volatilizetion losses characteristic of fluoaluminates:

(1) High tempe'raturelosses, i. e., sublimation a of metal fluorides and atmospheric oxidation and hydrolysisof constituent fluorides. Such losses are characteristic of all fluoaluminates, and occur in the case of natural cryollte to an appreciable extent at temperatures oi about 700 C. and high fluorine," volatilizable acid," and the like are X-ray, analyses 5 indicate that fluoaluminates consist in general of accompanyvolatilization of H20 upon application be construed as limiting the. scope'oi the inven employed inthis disclosure, it is to be understood that the reference is mainly to the second named of the two types of fluorine volatilization losses. The present invention is not directed to a process which eliminates fluorine losses of the first named type. h

The principal object of the present invention is to provide an inexpensive process by which a synthetic fiuoaluminate, such as cryolite and chiolite, which undergoes loss of fluorine upon heating, may be converted into a product which will not suffer an appreciable loss of fluorine at relatively high heat.

Still another object of the present invention is to provide a process whereby the objectionable l5 fuming features common to most synthetic fiuoaluminates is overcome and a product perfuming much like the purified natural material is I obtained.

I have found that volatilization of acid is parit allows formation of a highly granular and hence very filtrable product. Further, the use of highly acidic precipitating media causes many objectionable contaminating impurities to pass into so the mother liquor insteadof being occluded or co-precipitated with the solid product. Examples of such acid processes are cited in U. S. Patent No.1,873,'l27. A further object of the present invention, therefore, is to enable operation of desirable acid precipitation processes while realizing all the advantages of a product containing no volatilizable acidic fluorine compounds.

' Other objects will be apparent from a consideration of the specification and claims.

The process of the present invention comprises the treatment of a fiuoaluminate, characterized by loss of fluorine upon'heating, with an amount of alkaline material sufllcient to fix or stabilize the Such a 25 V precipitation process; however, is valuable since The excess of alkaline material employed is preferably suficient to insure complete fixing of volatilizable acid but may, if desired, be suilicient.

to make up for any deficiency of alkali metal in the product desired. Thus, it has been found that synthetic cryolite prepared by the acid processes previously. mentioned will usually lack sufiicient sodium to correspond to the-formula NaaAlFs. It is within the scope of the claims to fix or stabilize such a double fluoride and to so determine the quantity of alkaline material that the alkali metal content will substantially correspond to the formula of the desired compound or at least be brought nearer to it.

The optimum amount of alkaline material employed depends upon the particular alkali used, the composition or nature of the synthetic fluoaluminate, and the degree of residual alkalinity which may be tolerated in the resulting product. This optimum amount must be determined empirically. Generally, the' alkali is employed in several-fold excess of the amount stoichiometrically equivalent to the volatilizable acid. Thus, for example, the residual free alkali (as NazO),

found by acidimetric titration in products resulting from treating portions of a synthetic cryolite with (a) aqueous soda ash equivalent to 2.5% acid, calculated as HF, in the original cryolite, and (b) aqueous NaOH equivalent to-2.5% acid,

calculated as HF, in the original cryolite was (a) 3.17% and (b) 0.73%, after drying thev products at 115 C. In a similar experiment, NaOH equivalent to 1.5% acid, calculated as HF, was used to fix the acid and the residual free alkali was found to be 0.38% (as NazO') of caustic sufilced to fix the approximately 0.75% acid, calculated as HF, volatilizable at 600 C., but still smaller amounts were insufiicient.

There is no critical upper limit upon the amount of alkali employed other than that the amount-be insufilcient to render the product unacceptable for its intended use, the product being applicable for any of a wide variety of uses. Obviously, the amount of alkaline material mixed volatilizable fluorine compound so that upon hea'twith the inate will preferably be at least ing the treated product will not lose an appreciable amount of fluorine. The alkaline material is mixed with the fiuoaluminate after separation of the latter from the solution in which it is produced and before it has been subjected to temmixture, and in most cases, the amount will be peratures sufficiently high to effect volatilization of said fluorine. The alkaline material may bechosen from any one of-a number of hydroxides, oxides, or alkaline salts, such as the hydroxides or carbonates of the alkali metals, sodium or potassium aluminate, activated aluminum oxide,

tion of acidic fluorine compounds. Preferably, so

the alkaline material is selected with reference to the impurities which its use may add to the fluoaluminate. Thus, for example, sodium hydroxide is employed in preference to potassium suflicient to stabilize substantially the volatilizable fluorine compounds. In general, the NazO alkalimetric equivalent of the alkali material mixed with the fiuoaluminate will not be more than about 15 weight per cent. of the resulting lower than about 10%. A satisfactory product for use in the electrolytic production of aluminum results when the amount of alkaline material mixed with the 'fiuoalurninate is limited to a reasonably small value, suitably about 2% to 5%. on the stated basis. As an example, the "NazO alkalimetric equivalent" of 2.65 grams NaOH (i e., 2.05 grams NaaO) is 2% of the weight of the composition obtained by mixing 2.65 grams NaOH and- 100 grams cryolite. the N120 alkalimetric equivalent of 5 grams 02.0 is calculated to be about 5.54 grams (of New); Obviously, smaller amounts may be emhydroxide for the treatment of asynthetic cryo- 55 Ployed although with Very small amounts of lite.

The amount of alkaline materal added to the capable of reacting to an appreciable extent with the fluoaluminate .so that a strongly alkaline 1 product is not a necessary consequence of the use of a slight excess of alkaline material.

kaline material complete'stabilization may not be realized.

The alkaline material may be mixed with the finely divided dry fiuoaluminate or with a moist product, such as the filtered and washed fluoaluminate precipitate prior to drying, at which step in the process of the production of the fluoaluminate the product may contain from about 15% to water, generally in the neighbox-hood of 15% to 40%, water. In accordance This lesser amount As a further example,-

with the'present invention,- the alkaline material is' not added to the solution containing the ingredients which react to'rorm the fluoalumlnate or to the-solution containing the fluoaluminate precipitate in suspension, but as herein pointed out, it is mixed with the dry or moist fluoaluminate, that is, subsequent to the separation oi the fluoaluminate from the solution in which it is produced.- The alkaline material in finely divided dry form or in aqueous .or other solution may be mixed with the 'dry or moist fluoaluminate. For'example, when a dry product is treated, a dry alkaline material may be mechanically mixed therewith, and, ii desired, after 'the dry materials are mixed, water may be added thereto. A completely dry mix is not prefererd, however, since somewhat larger amounts of al-v kaline material must be employed and more attention must be given to uniformity tit mixing in order to prevent loss or fluorine upon heating, because the volatilisable fluorine is fixed at the time of its liberation at elevated temperatures,

i. e., the dry alkali does not penetrate the fluo-.

aluminate grains at low temperatures. In the case of the treatment of a moist fluoaluminate.

tageously in aqueous or'other solution to insure thorough mixing, but' dry alkaline material may be used, if desired. After the preparation or'the mixture of the fluoaluminate and the alkalinematerial, the product, if moist, may be dried at any commercially practical temperature, for example, 100-200 C.

The dry product, prepared from dry ingredients or in the presence of water and dried, may be distributed to the trade without further treatment and may be used industrially with assasa'r producing the substantially alkali-free material and the second furnishing the product with substantially no loss on ignition, as above stated, but,

since nothing is to be gained thereby, the use of a sufllclent temperature in a single heat-treat: V

ment to accomplish both purposes is 11-200mmended, when a product of the described prop- 1 erties is desired. It is also to be understood that a product which is not convertible into an alkali,

- free product at a'relatively low temperature may be subjected to theheat-treatment described to produce a product with substantially no loss on ignition.

The following examples are illustrative of thepresent invention and the treatment of cryolite has been selected as the preferred embodiment of the invention:

Example I To 100 grams or finely divided dry synthetic cryolite, about 7 grams (more or less depending I the alkaline material added thereto is edvanon the amount of volatilizable acid present) of dry finely divided soda ash are added. After intimately mixing the cryolite and soda ash, the product may be packaged and shipped to the trade. If desired, the mixture may be heated to a temperature in the range of 400 C. to 700 C.

for a period of time dependent upon the temperature employed and the percentage of volatilizable acid present. Theheating at lower temperatures within the range, for example from 400 C.

' to 500 0., usually requires about one hour. The

assurance that no appreciable amount or fluorine will be liberated upon heating.. In the case of this product, the losses in weight upon heating.

to a relatively high temperature are largelydue to carbon dioxide and/or water, little or no fluorine loss being encountered.

' 11' an excess of alkaline material has been used, as previously stated, itis capable. of reacting witlithe fluoaluminate. In such cases, the

product may be heated at a temperature as low as 100-125 C. when an alkali metal hydroxide has been employed, and at a temperature within the range 400 to 700 C. when an alkali carbonate has been employed'to obtain a substantially alkali-tree product,

The product, if heated at a temperature, will be capable oi withstanding relatively high heat-without substantial ignition loss,

time of treatment may be lessened by an increase in the temperature used. Most of the excess alkaline material reacts with the iluoaluminate and carbon dioxide and water are evolved. and

a very stable cryolite is produced in which the proportions of sodium, aluminum, and fluorine are substantially'those of natural cryolite.

' Example II To 100 grams (calculated on dry weight) of the filtered, washed, and moist synthetic cryolite,

l a suilicient amount of a strong solution of soda ash is added to equal about '7 grams of NaZCOa.

- Afterintimately mixing the cryolite and the sodav ash solution, the product is dried. The dried prod-- uct may be marketed, without further heating, or it may be subjected to the heat-treatment described in Example I, in which latter case, as

- fher'einbefore pointed out, the product may be used industrially at elevated temperatures with assurthat is to say, if such a product is desired, the

time-temperature conditions of heat-treatment are selected so that upon completion of the heattreatment, a product is produced which when higher, will compare favorably, with respect to any type volatilization loss, with natural cryolite.

. used industrially, for example, in the case of synthetic cryolite, at temperatures or l'00 C. or.

ance that the product will beycomparable to natural cryolite.

- Example III To 100 grains (calculated on the dry weight) of filtered, washed, and moist synthetic cryolite, a sufllcient amount of a strong aqueous solution of sodium hydroxide is added to adjust the mixture so that the weight ratio of NaOH to NaaAlFt is equal to 0.03 and so that the weight ratio 21 0-.

The use of, temperatures of about 400 C. or higher a up to'the melting point of the product, for example 400 C. to 700 0., will produce a product stantialiy alkali' iree material, 'thepro'duct may to H 0 plus NaaAlFs is about 0.18. The product is then dried-for about one hour at C. The alkalinity of the dried product 'will equal about 0.5% titratable NaOH and there is no acid loss upon heating; Ignition at 600 C. then eliminates waterand the product contains only sodium, aidminum,and fluorine in combination as stable as that in naturalcryolite.

I claim:

1. The process oftreating asub-divided, solid I synthetic fluoaluminate characterized by loss'of fluorine .upon heating, which comprises mixing an alkalinejmaterial'with said sub-divided, solid'synbe subjected to two heat-treatments, the flrst' thetic fluoaluminate subsequent to its separation from the solution in which itis produced and prior to its subjection to temperatures sufliciently high toefiect volatilization of said fluorine, the N320 equivalent of said alkaline material being not more than about 15 weight per cent. of the resulting mixture on-the dry basis. 3 e

2. The process of claim 1 wherein the amount of alkaline material mixed with the fluoaluminate is sufllcient to stabilize substantially said fluorine, and the NazO equivalent thereof is not more than about 10 weight per cent of'the resulting mixture. 1 3. The process of claim l wherein the synthetic fluoaluminate is synthetic cryolite, the alkaline material is selected from the group of sodium hydroxide andvsodiuin carbonate, and the amount of alkaline material mixed with 'the'fluoaluminate is suflicient to stabilize substantially said fluorine,

and the NazO equivalent thereof is not more than 1 about 5 weight per cent. of the resulting mixture.

4. The process of claim 1 wherein the synthetic fluoaluminate is synthetic cryolite, the alkaline material is sodium hydroxide, and the NazO equiv- 'alent of said sodium hydroxide is between about tion from the solution inrwhichit is produced and 1 prior to its subjection to temperatures sufllciently high to efiect volatilization of said fluorine, the

NaaO equivalent of said alkaline material being not more than about 15 weight percent. of the resulting mixture on the dry basis, and heating a the product at a temperature within the range 100 to 700 C.

6. The process of claim 5 wherein the amount of alkaline material mixed with the fluoaluminate is sumcient to stabilize substantially said fluorine and the NazQ equivalent thereof is not more than about weight percent.'of the resulting mixture fluoaluminate is synthetic cryolite, and the amount of alkaline material mixed with the fluoaluminate is sufllcient to stabilizesubstantially said fluorine and the NazO equivalent thereof is not more'than about 10 weight per cent. of the 50 v resulting mixture; i i

' to 700 C.

13. The process of claim l0vwherein the syn- '1. The process of claim S'Wherein the synthetic 8. The process of claim 5 whereinthe syntheti fluoaluminate is synthetic cryolite, and the alka line material is caustic soda, the NazO equiva lent of said caustic soda is between about 2% am about-5% by weight of the resulting mixture.

9. The process of claim 5 wherein the synthetii fluoaluminate is synthetic cryolite, the alkaline material is sodium carbonate, the amount of sodium carbonate, mixed with the fluoaluminate is lo sufllcient to stabilize substantially said fluorine 10.The process of treating asub-divided, solid synthetic fluoalumi iate characterized by loss of fluorine'upon heating, which comprises mixing an alkaline material in the presence of water with saidsub-divided, solid synthetic fluoaluminate o subsequent to its separation from the solution in which it is produced and prior to its subjection to temperatures sumciently high to efi'ectvolatilization of said fluorine, the Nazo equivalent of said alkaline material being not morethan about 15 weight 'per cent. of the resulting product on the dry basis, and dryingthe product.

IL- The process otclaim 10 wherein the amount of alkaline material is sumcient to stabilize substantially said fluorine and "the NazO equivalent thereof is not more than about 10 weight per cent. of the resulting mixture.

12. The process of claim 10 wherein the synthetic fluoaluminate is synthetic cryolite, the alkaline material is selected from the group consisting of sodium hydroxide and sodium carbonate, the amount of alkaline material mixed with the fluoaluminate is sufiicient to stabilize substantially said fluorine, and the NazO equivalent thereof'is not more than about 5 weight per cent of the resulting mixture, and the product is heated at a temperature within the range 100' thetic fluQa-luminate is synthetic cryolite, the

alkaline material -.is sodium hydroxide, and the NazO equivalent of said sodium'hydroxide isbe- CHARLES a. MILES.- 

